During conveying of milk with the help of an underpressure over a long distance and in particular through feedpipes, there exists the danger that the milk will change from its original state and spoil as a whole. This may, for example, be due to the fact that, during conveying, the thin albumen membranes which surround the fatty globules in the milk can be damaged mechanically, after which the unprotected fat can be attacked by fat-cleaving enzymes commonly called lipases. In this case, a relatively significant cleaving of various free fatty acids (FFA) takes place. Of these, the butter acid is in particular mentioned because of its rancid taste. Long-chain free fatty acids, for example stearic acid, can produce a soapy taste in the milk and in milk products. During the destruction of the fatty globules, free fat is also formed in addition to the free fatty acids, namely, fat which has lost its globule structure and behaves physically differently than the undamaged fatty globules. Since free fat has a greater tendency to be distributed unevenly, it makes it more difficult to pull a representative fat sample from the conveyed milk and to satisfactorily separate out the fat.
The amount of free fatty acids present in milk is usually given in terms of a milli-equivalent per liter of milk (mequ./1). In the case of values of between 1.5 and 2.0, depending on the composition of the free fatty acids, the consumer will typically be very aware of a poor taste of the milk or milk products. Therefore, in the case of milk which is to be transported from a farm, a free fatty acid content of not more than approximately 1.0 mequ./1 is desirable in order to leave an allowance for influences at the dairy, for example milk transport outside and within the dairy, the time period until pasteurization, and so forth.
As is known, lipases are always present in milk. However, the production of free fatty acids varies widely. It has been found that an increased amount of free fatty acids is often produced during late lactation, during relatively high milk output, when the cow is in heat or when a change in feed occurs. Furthermore, it has been found that the probability of production of free fatty acids during conveying of warm milk is substantially higher than during conveying of cold milk. The conditions under which a mechanical milking is carried out also appear to have a certain influence on the production of free fatty acids. All in all, the degree of free fatty acids which exist in the delivered milk obviously depends on a plurality of factors such as, for example, and aside from the sensitivity of the cows, the manner and carefulness of the milking routine, the quality of the milk cooling, the frequency of hauling away the milk, the design and maintenance of the milking apparatus, and many other factors. Depending on the presence and relative importance of these influencing factors, one finds FFA values in milk which has been supplied by a farm of from 0.4 up to more than 1.8 mequ./1.
One very important single factor has been found to be the type of milking apparatus utilized. If one compares various types of modern milking apparatuses of equal quality and design which are operated under equal conditions, then the average content of free fatty acids is approximately as follows:
______________________________________ Type mequ./l ______________________________________ Bucket milking apparatus 0.45 Pipe milking apparatus with a low milking 0.60 pipe (or low milk measuring cups) Pipe milking apparatus with an overhead 0.80 milking pipe (or overhead milk measuring cups) Milk in the udder of a cow 0.30 ______________________________________
From these values it becomes clear that a relatively large difference in the production of free fatty acids is present in a pipe milking apparatus with a low milking pipe as compared to one with an overhead milking pipe. A milking apparatus with a low milking pipe which has been designed and serviced normally will, during a good milking routine and even under unfavorable conditions like late lactation, a change in feeding and so forth, rarely come into the range of 1 mequ./1, but this happens relatively frequently in a modern apparatus with an overhead milking pipe. On the other hand, the milking apparatus with an overhead milk discharge pipe is very common in the classic family operation with a tie-up stall in most countries which produce milk.
The basic difference between a pipe-milking apparatus with an overhead pipe and one with a low pipe is that in the former the milk is conveyed, with the help of the underpressure or vacuum which exists in the milk discharge pipe, from the collecting piece of the milking tool through a long hose to the milk discharge pipe, which lies approximately 1.8 to 2 meters above the ground. In the case of the low milk discharge pipe, which is supported approximately at the height of the collecting piece, the upward conveying of the milk through a long milk hose is absent.
A milking machine actually has a dual purpose. It must on the one hand suck milk out of the teat, and on the other hand move the milk from the area near the teat to a storage reservoir. Both functions are accomplished by the suction force of the underpressure which exists in the milk discharge pipe. If the milk were conveyed as a continuous milk column to the milk discharge pipe, the milking vacuum which exists at the tip of the teat and with which the milk is sucked out of the teat would, especially due to hydrostatic pressure losses, become very unstable at increased milk flow rates and would decrease too far below half of the nominal vacuum. Such a process would result in unacceptable problems like falling off of the milking tool, extremely long milking times, and very poor milking completion. Furthermore, the milking vacuum would be exposed in addition to extremely cyclic variations due to the pump action of the rubber teat, which opens and closes rhythmically in connection with the mass forces of the conveyed milk. The use of such a milking vacuum at the teat would be totally impossible, both from the viewpoint of udder health and also the efficiency of the milking operation. For these reasons, standard modern milking machines each feed small amounts of air into the milking tool (approximately 8 liters per minute of air). Through this, the milk column is physically interrupted and thus lighter and more pressure elastic. In this manner, the hydrostatic pressure losses and the cyclic vacuum variations at the teat can be reduced to a tolerable degree; and only in this manner are the conditions created which make milking in a practical manner possible at all, particularly with an overhead milking pipe which is disposed approximately 2 meters high.
If the raw milk does not contain any finely distributed air and if it is cooled in addition, then an enormously high mechanical stress is needed in order to cause an increase in the FFA content which is worth mentioning. If the conveying air is intensively mixed with milk, however, three primary concerns exist with respect to the FFA content:
1. The fatty globule membranes will have a greatly increased tendency to burst along the boundary layer between the milk and the air, due to the surface tension and loading, and a fine and intensive mixing of air in the milk substantially increases the total area of this boundary surface. PA0 2. Adding air also increases the flow rate of the discharged milk. This could cause, under particularly unfavorable conditions such as a high addition of air, narrow pipe cross sections, sharp transitions and so forth, impact and shearing forces between the moving milk particles themselves and between the moving milk particles and the pipe wall which are so high that direct destruction of the fatty globule membranes is possible. PA0 3. If milk containing air is guided through a centrifugal pump, a considerable damage of the albumen membranes can occur. For this, just a small percentage of air which cannot even be seen with the naked eye is sufficient. PA0 1. The amount of free fatty acids produced can be noticeably reduced. PA0 2. A smaller amount of free fat is produced, thereby providing better capability for separating the fat and easier removal of a representative fat sample from a milk storage reservoir. PA0 3. A reduced formation of foam in the milk is achieved. This facilitates better hygiene in the milk storage reservoir, a reduced danger of the milk storage reservoir overflowing, and a simpler reading of the amounts of milk in the reservoir with the help of measuring devices. PA0 4. The vacuum which exists during the various phases of the milking process is noticeably stabilized. PA0 5. The danger that mastitis agents will be transmitted from one cow to another by milk returning from the milk discharge pipe when the other cow is just being milked dry is reduced considerably.
A basic purpose of the present invention is therefore to provide an apparatus of the above-mentioned type in which the milk is guided through feedpipes but in which the FFA values are not significantly increased as the milk is conveyed.
To explain the general thought of the invention, it is first pointed out that it was found that in an inclined pipe, a milk and air mixture can flow in two layers, the heavier milk moving at the bottom in the pipe and the air passing over it. The milk in this case flows due to the force of gravity.
In a feedpipe however, the conveying of the milk can occur only through a pressure difference between the upper and the lower end of a milk column or one or more milk plugs. A plug formation is therefore necessary in a feedpipe during the conveying method with the help of an underpressure. However, this has the result that the inside diameter of the feedpipe cannot be chosen too large, even though, due to the need to minimize flow losses and thus vacuum losses in the milking tool, it is absolutely necessary to strive for an inside diameter of the feedpipe which is as large as possible. It has now been determined that, at cross sections of the feedpipe which are too large or at milk flow rates which are too small, the conveying of the milk no longer occurs smoothly and steadily because, under these conditions, proper milk plug formation becomes increasingly difficult. The conveying air which is added to the milk breaks increasingly under these conditions through the milk plug which is to be moved, and the plug residues flow back along the feedpipe until a new plug is formed or until all the milk as drops or as droplets is carried whirlingly along through the upwardly flowing conveying air.
In addition, since the amount of conveying air added is practically the same for each milking phase, there results as an additional complication the fact that the added amount of air is generally chosen so that, for maximum milk flow, a favorable discharge of the milk is assured. Thus, in this particular situation, a balance exists between the milk flow and the amount of conveying-air added. At the start and end of a milking cycle, however, the milk flow is substantially less than the maximum milk flow. In the case of a typical milk flow curve, the peak milk flow is relatively short. For example, during 50% of the milking time, the milk flow typically reaches only about 10%-20% of the maximum milk flow. During late lactation or, for example, because of uneven milking intervals (during evening milking), significantly smaller milk amounts are milked, causing the unbalance to be further sharpened. A greatly increased danger of dissolving the milk plugs and of conveying a pure air-milk mixture exists in these cases.